This invention relates to accelerator beam optics and, more particularly, to shaping charged particle beams for confining the beam energy distribution over a selected target area.
Charged particle beams contain particles which have been accelerated to high energies. The accelerated particles are directed onto target areas for a variety of purposes, e.g. medical treatments, X-ray generation, lithography, neutron spallation, tritium production, food sterilization, etc. In some applications a target area large compared to an original beam must be illuminated by the beam, where confinement of the beam energy and/or the uniformity of illumination, i.e., energy deposition, on the target area are important.
For example, in the production of tritium, an intense linear accelerator (linac) produces a proton beam (i.e., 400 mA at 2 GeV or about a gigawatt of continuous power) which is incident on a target composed of lead to generate neutrons which, in turn, strike lithium for tritium production. A beam target area of 4 m by 4 m may be required to be covered by a beam having only millimeter dimensions at the linac.
A simple beam expansion by conventional linear magnetic optics is not appropriate. The Gaussian-like intensity distribution of particles in the beam would conventionally provide a sharply peaked beam at the target with an appreciable peak intensity at several times the beam rms radius. Further, large amounts of the beam energy are contained in the beam fringes which would not be available over the target area if the beam is to provide any substantial energy deposition on the target edges. Another approach would be to raster the beam over the target area. If a typical beam is expanded to a diameter of several centimeters and rastered over the above 16 m.sup.2 surface area in a two-dimensional scan, thermal considerations dictate a minimum scanning frequency of about 1 Hz in the slower scan direction and about 10 Hz in a direction normal to the slower scan direction. Moreover, large magnets with very large apertures would be needed and the peak reactive EMF's would be large. Additionally, the magnets would need to be energized with several harmonics or provided with high power swithing devices to maintain a uniform target illumination.
It would be desirable to form the beam where the beam energy is confined over a large area and with a core beam area of relatively uniform charged particle intensity for uniform energy deposition on a target area. This problem is addressed by the present invention wherein confined particle beams define relatively uniform beam intensity distributions as large area beams or ribbon beams for impacting a target area.
Accordingly, it is one object of the present invention to form a charged particle beam having a nonuniform, peaked particle distribution, e.g., a Gaussian or parabolic distribution, into a particle beam having a confined energy distribution in at least one dimension.
It is another object of the present invention to form a charged particle beam to a configuration for relatively uniform energy deposition in at least one dimension over a target area.
One other object of the present invention is to use nonlinear magnetic optical elements to form a well contained charged particle beam having a relatively uniform particle distribution within beam edge portions.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which folows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.